11,817 research outputs found
Thermodynamics and Kinetics of Folding of a Small Peptide
We study the thermodynamics and kinetics of folding for a small peptide. Our
data rely on Monte Carlo simulations where the interactions among all atoms are
taken into account. Monte Carlo kinetics is used to study folding of the
peptide at suitable temperatures. The results of these canonical simulations
are compared with that of a generalized-ensemble simulation. Our work
demonstrates that concepts of folding which were developed in the past for
minimalist models hold also for this peptide when simulated with an all-atom
force field
Final spins from the merger of precessing binary black holes
The inspiral of binary black holes is governed by gravitational radiation
reaction at binary separations r < 1000 M, yet it is too computationally
expensive to begin numerical-relativity simulations with initial separations r
> 10 M. Fortunately, binary evolution between these separations is well
described by post-Newtonian equations of motion. We examine how this
post-Newtonian evolution affects the distribution of spin orientations at
separations r ~ 10 M where numerical-relativity simulations typically begin.
Although isotropic spin distributions at r ~ 1000 M remain isotropic at r ~ 10
M, distributions that are initially partially aligned with the orbital angular
momentum can be significantly distorted during the post-Newtonian inspiral.
Spin precession tends to align (anti-align) the binary black hole spins with
each other if the spin of the more massive black hole is initially partially
aligned (anti-aligned) with the orbital angular momentum, thus increasing
(decreasing) the average final spin. Spin precession is stronger for
comparable-mass binaries, and could produce significant spin alignment before
merger for both supermassive and stellar-mass black hole binaries. We also
point out that precession induces an intrinsic accuracy limitation (< 0.03 in
the dimensionless spin magnitude, < 20 degrees in the direction) in predicting
the final spin resulting from the merger of widely separated binaries.Comment: 20 pages, 16 figures, new PN terms, submitted to PR
Virtual Resonant States in Two-Photon Decay Processes: Lower-Order Terms, Subtractions, and Physical Interpretations
We investigate the two-photon decay rate of a highly excited atomic state
which can decay to bound states of lower energy via cascade processes. We show
that a naive treatment of the process, based on the introduction of
phenomenological decay rates for the intermediate, resonant states, leads to
lower-order terms which need to be subtracted in order to obtain the coherent
two-photon correction to the decay rate. The sum of the lower-order terms is
exactly equal to the one-photon decay rate of the initial state, provided the
naive two-photon decay rates are summed over all available two-photon channels.
A quantum electrodynamics (QED) treatment of the problem leads to an
"automatic" subtraction of the lower-order terms.Comment: 8 pages, RevTe
Magnetic phase diagram of
Magnetometry, electrical transport, and neutron scattering measurements were
performed on single crystals of the Fe^{4+}-containing perovskite-related phase
Sr_3Fe_2O_7-x as a function of oxygen content. Although both the crystal
structure and electron configuration of this compound are closely similar to
those of well-studied ruthenates and manganates, it exhibits very different
physical properties. The fully-oxygenated compound (x=0) exhibits a
charge-disproportionation transition at T_D = 340 K, and an antiferromagnetic
transition at T_N = 115 K. For temperatures T \leq T_D, the material is a
small-gap insulator; the antiferromagnetic order is incommensurate, which
implies competing exchange interactions between the Fe^{4+} moments. The
fully-deoxygenated compound (x=1) is highly insulating, and its Fe^{3+} moments
exhibit commensurate antiferromagnetic order below T_N ~ 600 K. Compounds with
intermediate x exhibit different order with lower T_N, likely as a consequence
of frustrated exchange interactions between Fe^{3+} and Fe^{4+} sublattices. A
previous proposal that the magnetic transition temperature reaches zero is not
supported.Comment: 8 pages, 6 figure
Quantitative trait locus analysis of parasitoid counteradaptation to symbiont-conferred resistance.
Insect hosts and parasitoids are engaged in an intense struggle of antagonistic coevolution. Infection with heritable bacterial endosymbionts can substantially increase the resistance of aphids to parasitoid wasps, which exerts selection on parasitoids to overcome this symbiont-conferred protection (counteradaptation). Experimental evolution in the laboratory has produced counteradapted populations of the parasitoid wasp Lysiphlebus fabarum. These populations can parasitize black bean aphids (Aphis fabae) protected by the bacterial endosymbiont Hamiltonella defensa, which confers high resistance against L. fabarum. We used two experimentally evolved parasitoid populations to study the genetic architecture of the counteradaptation to symbiont-conferred resistance by QTL analysis. With simple crossing experiments, we showed that the counteradaptation is a recessive trait depending on the maternal genotype. Based on these results, we designed a customized crossing scheme to genotype a mapping population phenotyped for the ability to parasitize Hamiltonella-protected aphids. Using 1835 SNP markers obtained by ddRAD sequencing, we constructed a high-density linkage map consisting of six linkage groups (LGs) with an overall length of 828.3 cM and an average marker spacing of 0.45 cM. We identified a single QTL associated with the counteradaptation to Hamiltonella in L. fabarum on linkage group 2. Out of 120 genes located in this QTL, several genes encoding putative venoms may represent candidates for counteradaptation, as parasitoid wasps inject venoms into their hosts during oviposition
Generalized Ensemble and Tempering Simulations: A Unified View
From the underlying Master equations we derive one-dimensional stochastic
processes that describe generalized ensemble simulations as well as tempering
(simulated and parallel) simulations. The representations obtained are either
in the form of a one-dimensional Fokker-Planck equation or a hopping process on
a one-dimensional chain. In particular, we discuss the conditions under which
these representations are valid approximate Markovian descriptions of the
random walk in order parameter or control parameter space. They allow a unified
discussion of the stationary distribution on, as well as of the stationary flow
across each space. We demonstrate that optimizing the flow is equivalent to
minimizing the first passage time for crossing the space, and discuss the
consequences of our results for optimizing simulations. Finally, we point out
the limitations of these representations under conditions of broken ergodicity.Comment: 11 pages Latex, 2 eps figures, revised version, typos corrected, PRE
in pres
This elusive objective existence
Zurek's existential interpretation of quantum mechanics suffers from three
classical prejudices, including the belief that space and time are
intrinsically and infinitely differentiated. They compel him to relativize the
concept of objective existence in two ways. The elimination of these prejudices
makes it possible to recognize the quantum formalism's ontological implications
- the relative and contingent reality of spatiotemporal distinctions and the
extrinsic and finite spatiotemporal differentiation of the physical world -
which in turn makes it possible to arrive at an unqualified objective
existence. Contrary to a widespread misconception, viewing the quantum
formalism as being fundamentally a probability algorithm does not imply that
quantum mechanics is concerned with states of knowledge rather than states of
Nature. On the contrary, it makes possible a complete and strongly objective
description of the physical world that requires no reference to observers. What
objectively exists, in a sense that requires no qualification, is the
trajectories of macroscopic objects, whose fuzziness is empirically irrelevant,
the properties and values of whose possession these trajectories provide
indelible records, and the fuzzy and temporally undifferentiated states of
affairs that obtain between measurements and are described by counterfactual
probability assignments.Comment: To appear in IJQI; 21 pages, LaTe
Two-Photon Decays Reexamined: Cascade Contributions and Gauge Invariance
The purpose of this paper is to calculate the two-photon decay rate
corresponding to the two-photon transitions nS->1S and nD->1S in hydrogenlike
ions with a low nuclear charge number Z (for principal quantum numbers n =
2,...,8. Numerical results are obtained within a nonrelativistic framework, and
the results are found to scale approximately as (Z alpha)^6/n^3, where alpha is
the fine-structure constant. We also attempt to clarify a number of subtle
issues regarding the treatment of the coherent, quasi-simultaneous emission of
the two photons as opposed to one-photon cascades. In particular, the gauge
invariance of the decay rate is shown explicitly.Comment: 10 pages, LaTe
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